Generally, an automatic transmission for vehicles includes a torque converter which is a fluid link between an engine and the transmission acting as both a torque multiplier and fluid coupling, a multi-stage gear shift mechanism connected to the torque converter, and a plurality of friction elements actuated by hydraulic pressure and which select and control one gear stage of the gear shift mechanism depending on road load and engine throttle opening of the vehicle.
The friction elements are provided enabling control of each gear stage of the multi-stage gear shift mechanism, comprised of a planetary gearset, and the friction elements are selectively activated/deactivated by the hydraulic control system which controls the hydraulic pressure generated by an oil pump, and, as a result, shift ratios of the planetary gearsets are changed to realize automatic shifting.
Also, the friction elements are selectively operated by the changing of hydraulic pressure directional flow by a plurality of valves provided in the hydraulic control system. A position of a manual valve is indexed with a driver selector lever position to realize port conversion, receives hydraulic pressure from the oil pump, and communicates with a line to supply the hydraulic pressure to a shift control valve.
The above hydraulic control system for automatic transmissions includes first to fourth shift mechanisms and first to ninth friction elements. A plurality of shift ranges can be realized by selectively operating the friction elements.
However, with the conventional hydraulic control system, because line pressure is reduced during N to D control, and shifting into the low L range is done mechanically, operational pressure of the operating friction element is reduced and the friction element slips such that control precision and responsiveness of the friction element is reduced.